Transparent armor is a sub-set of ballistic armor. Ballistic armor is generally designed to provide protection of personnel and equipment from ballistic projectiles, explosive ordnance, and forces and objects from detonation of improvised explosive devices (collectively hereinafter “projectiles”). One purpose for ballistic armor, including transparent armor, is to provide a means of disbursing the kinetic energy of projectiles to prevent them from reaching their target. Although this may be accomplished by interposing a large mass of any of a number of different materials between the target and the incoming projectile, experience has shown that a much more efficient means of energy disbursement is provided by suitably engineered ballistic armor structures wherein layers of different materials act to disrupt and disperse the energy of an incoming projectile. Such structures strive to maximize the amount of material which may be acted upon to absorb and disburse the energy of the projectile, while at the same time breaking or deforming the projectile and distributing these resulting fragments into a wider area. Such structures further strive to minimize the total amount of materials required for the protection of a specific area.
Ballistic armor structures generally contain one or more layers of material engineered to spread the force of the impact by deforming, deflecting, or fragmenting the ballistic projectile while the ballistic armor itself may undergo deformation or localized fragmentation. The deformation and localized fragmentation processes of the ballistic armor structure absorb a large portion of energy from the projectile while simultaneously spreading the impacted area to involve more material in successive layers. Both hardness and toughness of the ballistic armor structure are required for these functions.
In the field of ballistic armor structures, the initial layer of material used to disrupt the incoming ballistic projectile is often referred to as the “strike face”, or alternatively, the “hard face”. The strike face is typically a layer of relatively hard and tough material designed to deform, and in some cases fragment or blunt, to absorb at least some of the energy of the incoming projectile, thereby distributing the projectile's energy. Following the strike face, there are commonly other layers specifically designed to absorb the remaining energy of the impacting material and pieces of the previous strike face. These layers are, often referred to as the “backing” or “back layers”.
The process of energy absorption and disbursement of the incoming projectile by the ballistic armor structure is generally intended to result in deformation, displacement and/or localized fracture of the strike face, and deformation and/or displacement of the baking, but without penetration through the ballistic armor structure by any fragments of the ballistic projectile. Selection of materials for these distinct functions and attention to construction and coupling of the various layers is often used to optimize performance of the ballistic armor structure for an intended use.
Great advances have been made in selection of materials for optimizing the performance of ballistic armor structures. Use of high-strength, hard, and in some cases “tough” ceramics like aluminum oxide, boron carbide, titanium diboride and silicon carbide for the strike face; and rigid or soft laminates of fibrous materials such as fiberglass, aramid, or polyethylene fiber for the backing have greatly reduced the mass and bulk of protective structures. These advances, unfortunately, have not been readily applicable to those areas where a transparent protective structure is required. The high-strength, hard ceramics and the laminated fibrous backing materials are not transparent; and so none are adaptable to transparent protective structures.
The need for transparency in transparent armor substantially limits possible choices of materials for fabrication of the strike face of transparent protective systems, especially if aerial density is a factor. Although recent advances have been demonstrated in use of crystalline or highly processed materials such as, hot-pressed spinel, aluminum oxynitride (ALON) ceramics, or melt grown aluminum oxide (sapphire) crystal sheets for the strike face, manufacturing cost and other limitations would seem to restrict their use in many applications, or to limit their utilization to serving as just a strike face layer in an overall transparent armor system.
One common material used for fabrication of the strike face in transparent armor is borosilicate float glass or soda lime glass, a material which is neither very hard, nor very tough, and which has a relatively high specific density. This results in the need to greatly increase the aerial mass and bulk of transparent armors in order to preserve effectiveness against projectiles. Such increase in aerial mass and bulk ultimately results in a conventional transparent armor having an increased weight per level of protection provided by the transparent armor.
A similar situation exists in regard to the materials used in the backing layers. The fibrous laminates traditionally used in the backing layers of ballistic armor structures are not transparent. Traditional backing and fragment catching layers for transparent armor are often un-reinforced sheets of polyacrylic or polycarbonate polymer, although some advances have been made in the use of optimized copolymer compositions for these layers. Thus, for most transparent armor applications, the common solution is a strike face of multiple layers of borosilicate float glass with a backup layer or layers of a polymer sheet to catch fragments, bound together with a conventional transparent adhesive.
Another approach has been to utilize a strike face layer of one of the harder materials, such as a spinel, an ALON ceramic or a sapphire and to use borosilicate float glass or soda lime glass in the backing layers. However, even this approach has limitations when the transparent armor system is challenged by a large and/or high velocity projectile or by multiple hits from multiple projectiles.
When an ordinary glass plate, such as those used in the previously-developed transparent armor systems described above, is impacted by a high velocity projectile, fractures may be created at both the impacted surface as well as the back surface. The front surface cracks are Hertzian in origin where the response of the material varies with the extent of penetration. During penetration, the impacted material is compressed as well as sheared aside. At the edge of the plastically densified region, the (radial) “bearing stress” often creates a ring crack which commonly propagates into a fracture cone through the thickness. At greater indentation, the plastic shearing often generates hoop stresses which, in turn, can give rise to radial cracks (“Palmqvist cracks”) originating from the edge of the sheared zone on the top surface. Median cracks commonly appear at the lower edges of the densified region. During elastic recovery after projectile impact, the Palmqvist cracks and the median cracks commonly meet up to produce “half-penny” cracks. Additionally, lateral cracks at the lower edge of the densified region are often generated. When impacted by a blunt projectile, material under the impact in a transparent armor system may densify without much shearing. The traveling stress wave on the surface (“Rayleigh wave”) often generates high radial stresses which progressively produce concentric cracks.
The compression and shear waves that travel in-line through the glass in the back layers of the transparent armor reflect from the back surface to become tensile waves. These, then, produce tensile stresses which are the highest at the back surface of the last non-polymer ply in the transparent armor. The glass is bent outwards, often leading to large cracks generated at the back surface. In glass having a modulus of rupture (MOR) of 35 MPa, as with glasses in previously developed transparent armor system, it is no surprise that, in the baseline configurations made using seven plys of 9.5 mm soda lime silicate (SLS) float glass (e.g., STARFIRE® by PPG), nearly all of the back glass plys usually show extensive cracking which, in turn, greatly reduces visibility.
A substitute material to SLS could potentially be of great benefit in terms of maintaining integrity of the back plys to maintain transparency in a transparent armor system challenged by a large and/or high velocity projectile or by multiple hits from multiple projectiles. In addition, it would be useful to provide a transparent armor system that reduces areal density (AD). Such a transparent armor system would be particularly useful if it could be achieved without merely resorting to utilizing only the costly harder materials, such as spinel, ALON ceramic or sapphire.
Given the foregoing, what is needed are transparent armor systems with improved properties to preserve transparency in a transparent armor system challenged by a large and/or high velocity projectile or by multiple hits from multiple projectiles, without the above-identified limitations of previously-developed transparent armor systems.